1. Field of the Invention
The present invention relates to a battery.
2. Description of Related Art
Various types of batteries have been developed in recent years. For example, a nickel-metal hydride storage battery has rapidly come into wide use as a secondary battery with a high energy density and a high reliability.
This nickel-metal hydride storage battery has been known as e.g. a rectangular sealed nickel-metal hydride storage battery comprising an electrode plate assembly that includes a plurality of positive electrode plates and a plurality of negative electrode plates alternately laminated with separators interposed one by one between them, and a rectangular parallelepiped battery case containing the electrode plate assembly (e.g., JP-A-2001-313066, JP-A-2001-93505, and JP-A-2002-260719).
In the above nickel-metal hydride storage battery, the positive electrode plate includes for example a positive electrode plate produced in a manner that a positive electrode substrate made of foamed nickel (made by plating nickel on a skeleton surface of an urethane foam and then burning the urethane foam) is filled with a mix material for positive electrode (hereinafter, “positive mix material”) containing a positive active material.
Meanwhile, the foamed nickel is concretely made by the method in which a foamed urethane substrate is coated with a thin electroless nickel plating and then immersed in an electrolytic nickel plating solution in which a predetermined mount of current is supplied to a front-surface-side electrode placed facing the front surface of the foamed urethane substrate and a back-surface-side electrode placed facing the back surface of the foamed urethane substrate, thereby coating the surface of a skeleton constituting the foamed urethane substrate with the electrolytic nickel plating. Consequently, a nickel layer formed on the urethane skeleton of a middle portion of the foamed urethane substrate in its thickness direction is apt to be thinner than a nickel layer formed on the urethane skeleton of an upper part of the foamed urethane substrate closer to the front surface thereof and a nickel layer formed on the urethane skeleton of a lower part of the same closer to the back surface thereof. For example, an average thickness of the nickel layer in the middle portion would be about 50% of that of the nickel layer in the upper or lower part. After the foamed urethane is burned off, a foamed nickel obtained would have a thinner nickel layer in the middle portion (e.g. an average thickness of about 50%) than those in the parts close to the front surface and the back surface.
In the case where the positive electrode substrate is made of the foamed nickel, having a thin nickel layer in the middle portion as above, the positive electrode plate (the positive electrode substrate) will have a low current collecting property in the middle portion. The positive electrode substrate could not entirely have a good current collecting property. To solve such problem, there has been known a technique using nickel hydroxide powder that is to be filled in pores (a void part) of the positive electrode substrate and has a small particle diameter (e.g. a particle diameter of 5 μm or less) in relatively large amounts (e.g. 20 wt % or more) in order to provide a wide particle size distribution with an average particle diameter of about 10 μm (see JP-A-6(1994)-349489). By using this technique, the gaps between the nickel hydroxide particles each having a large particle diameter are filled with the small-diameter nickel hydroxide particles to increase a filling amount of the nickel hydroxide particles while providing a good conductive path between the nickel hydroxide particles to enhance the current collecting property of the positive electrode plate.
However, the nickel hydroxide will change to inert γ-type nickel oxyhydroxide in association with repeated charge and discharge cycles. Increased γ-type nickel oxyhydroxide may cause expansion of the positive electrode plate and decrease of an electrolyte in each separator, which shortens the battery life. In particular, the nickel hydroxide particles each having a small particle diameter (e.g. a particle diameter of 5 μm or less) are likely to change to γ-type nickel oxyhydroxide in association with the repeated charge and discharge. Accordingly, in the case where the nickel hydroxide powder containing a relatively large amount (e.g. 20 wt % or more) of the small-diameter nickel hydroxide particles (e.g. a particle diameter of 5 μm or less) as mentioned above, the γ-type nickel oxyhydroxide is apt to particularly increase due to repeated charge and discharge, which may significantly shorten the battery life.